Concept:The sentence requires a singular subject that matches the negative idea of doing something impossible or insane.Explanation:The phrase “in his right senses” refers to a sane person.Swallowing live coals is not a sensible action.So, the correct subject must mean “nobody.”“No one” is singular and expresses that not a single person would do this.The completed sentence is:“No one in his right senses will swallow live coals.”Answer:Noone